The most common measuring stick when evaluating political candidates is the conservative-to-liberal scale. Yet when it comes to the US Senate, a metric that often matters as much runs from small to large.

Massachusetts has usually had big figures in the US Senate. Love them or loathe them, that was true of both Ted Kennedy and John Kerry, just as it was true of Ed Brooke and Paul Tsongas, of Leverett Saltonstall and both Henry Cabot Lodges.

They were all big, nationally known and respected senators, men unafraid to speak their minds.

So today let’s look at the possible field through the prism of real or potential stature.

On the Democratic side, Ed Markey starts with a clear political height advantage over Steve Lynch. Yes, Markey is a sometimes quirky figure, with some goofy verbal tics (hmm, OK, you know?). Still, he’s well-versed in policy and well-regarded in Washington, and has a long record of accomplishments on a wide range of issues.

Lynch, more of a loner, is someone who goes small on big votes. Take, for example, Obamacare. He voted for the original House legislation, against the final bill on the crucial vote, then in favor of the reconciliation legislation essential to its passage. The explanation Lynch offered for that transparent attempt to have things both ways didn’t just strain credulity, but left it in shreds.

He also went small on the bank bailout. Voting no, as he did, was easy — and yet, many experts will tell you that without the federal infusion of cash, our entire financial system would have frozen up, with devastating consequences.

Then there’s his often irascible interpersonal style. During the health care debate, for example, Lynch’s peevish manner managed to alienate both Nancy Pelosi, then the House speaker, and Vicki Kennedy, who was trying to round up votes for her deceased husband’s lifelong cause.

Now for the Republicans. Scott Brown said Friday he’s not running. Three years ago, just-elected Senator Scott Brown was viewed as a rising Republican star nationally. But Brown didn’t have the skills or stature to hold the seat. More a follower than a leader, he often decided late how to vote on controversial matters, and thus did little to shape either public or Senate opinion. When it comes to the press, meanwhile, Brown seems scared of his own shadow. Further, as we saw in last year’s campaign, an off-putting pettiness pervades his entire operation, from Brown on down.

By contrast, Bill Weld, who has left the door slightly ajar, is a large figure, an iconoclastic thinker who doesn’t mind sailing into the wind. Perhaps because of his healthy ego, Weld also has a famously thick skin. As governor, he enjoyed, and excelled at, the back-and-forth with the press. He’s big in the way of Massachusetts Republicans of old, a man who would be a nationally important senator almost immediately upon taking office.

Another possible candidate is former Lieutenant Governor Kerry Healey. As Romney’s number two, Healey found herself in a tough position when her boss began a rightward evolution that alienated Massachusetts voters and clearly hurt her 2006 gubernatorial campaign. She made a big mistake in that campaign by allowing her consultants to go with a visceral attack ad that lent her campaign an ugly undertone. Still, Healey put some interesting ideas on the table, mixed it up with the press, and showed a zest for political combat. Although something of an enigma, at core Healey seems like a smart, engaging, data-oriented moderate with real growth potential.

Campaigns, of course, are about shaping, and reshaping, images, which means those starting-line impressions may well change — which would be good news indeed for several of the candidates.